Enea Pagliano

Negative chemical ionization GC/MS determination of nitrite and nitrate in seawater using exact matching double spike isotope dilution and derivatization with triethyloxonium tetrafluoroborate.

The alkylation of nitrite and nitrate by triethyloxonium tetrafluoroborate allows determination of their ethyl esters by headspace gas chromatography/mass spectrometry (GC/MS). In the present study, significant improvement in analytical performance is achieved using negative chemical ionization providing detection limits of 150 ng/L for NO(2)(-) and 600 ng/L for NO(3)(-), an order of magnitude better than those achieved using electron impact ionization. The derivatization procedure was optimized and alkaline conditions adopted to minimize conversion of nitrite to nitrate (determined to be 0.07% at 100 mg/L NO(2)(-)) and to avoid the exchange of oxygen between the analytes and the solvent (water). Quantitation entails use of isotopically enriched standards (N(18)O(2)(-) and (15)NO(3)(-)), which also permits monitoring of potential conversion from nitrite to nitrate during the analysis (double spike isotope dilution).

Reduction of measurement uncertainty by experimental design in high-order (double, triple, and quadruple) isotope dilution mass spectrometry: application to GC-MS measurement of bromide.

Since its introduction a century ago, isotope dilution analysis has played a central role in developments of analytical chemistry. This method has witnessed many elaborations and developments over the years. To date, we have single, double, and even triple isotope dilution methods. In this manuscript, we summarize the conceptual aspects of isotope dilution methods and introduce the quadruple dilution and the concept of exact matching triple and quadruple dilutions. The comparison of isotope dilution methods is performed by determination of bromide ions in groundwater using novel ethyl-derivatization chemistry in conjunction with GC/MS. We show that the benefits of higher-order isotope dilution methods are countered with a greater need for careful experimental design of the isotopic blends. Just as for ID2MS, ID3MS and ID4MS perform best when the isotope ratio of one sample/spike blend is matched with that of a standard/spike blend (exact matching).

High-precision quadruple isotope dilution method for simultaneous determination of nitrite and nitrate in seawater by GCMS after derivatization with triethyloxonium tetrafluoroborate.

Quadruple isotope dilution mass spectrometry (ID(4)MS) has been applied for simultaneous determination of nitrite and nitrate in seawater. ID(4)MS allows high-precision measurements and entails the use of isotopic internal standards ((18)O-nitrite and (15)N-nitrate). We include a tutorial on ID(4)MS outlining optimal experimental design which generates results with low uncertainties and obviates the need for direct (separate) evaluation of the procedural blank. Nitrite and nitrate detection was achieved using a headspace GCMS procedure based on single-step aqueous derivatization with triethyloxonium tetrafluoroborate at room temperature. In this paper the sample preparation was revised and fundamental aspects of this chemistry are presented. The proposed method has detection limits in the low parts-per-billion for both analytes, is reliable, precise, and has been validated using a seawater certified reference material (MOOS-2). Simplicity of the experimental design, low detection limits, and the use of quadruple isotope dilution makes the present method superior to the state-of-the-art for determination of nitrite and nitrate, and an ideal candidate for reference measurements of these analytes in seawater.

Novel Ethyl-Derivatization Approach for the Determination of Fluoride by Headspace Gas Chromatography/Mass Spectrometry

We report a novel derivatization chemistry for determination of fluoride based on the batch reaction of fluoride ions with triethyloxonium tetrachloroferrate(III) in a closed vessel to yield fluoroethane. Gaseous fluoroethane was readily separated from the matrix, sampled from the headspace, and determined by gas chromatography/mass spectrometry. The method was validated using rainwater certified reference material (IRMM CA408) and subsequently applied to the determination of fluoride in various matrixes, including tap water, seawater, and urine. An instrumental limit of detection of 3.2 μg/L with a linear range up to 50 mg/L was achieved. The proposed derivatization is a one-step reaction, requires no organic solvents, and is safe, as the derivatizing agent is nonvolatile. Determination of fluoride is affected by common fluoride-complexing agents, such as Al(III) and Fe(III). The effect of large amounts of these interferences was studied, and the adverse effect of these ions was eliminated by use of the method of standard additions.

Solution to the isotope dilution challenge

Over the years, isotope dilution mass spectrometry (IDMS) has witnessed many reformulations. One of the earliest expressions for this quantitation model was given by the single isotope dilution (IDMS). In IDMS, the mass fraction of an analyte A is obtained from mass spectrometric analysis of a mixture (blend) of sample and an isotopically-enriched form of the analyte (B) which acts as an internal standard:

Determination of thiocyanate in saliva by headspace gas chromatography-mass spectrometry, following a single-step aqueous derivatization with triethyloxonium tetrafluoroborate

A novel method for the determination of salivary thiocyanate is presented. Thiocyanate was converted into ethyl thiocyanate by single-step aqueous derivatization based on triethyloxonium tetrafluoroborate and measured by gas chromatography-mass spectrometry (15 min runtime). The ethyl thiocyanate derivative is volatile and can be sampled from the headspace. The derivatization chemistry proposed allows for separation of the analyte from saliva matrix whose introduction in the measurement system is avoided. Quantitation of the analyte was obtained by isotope dilution, employing a (13)C-enriched thiocyanate as internal standard. Technical details and fundamental aspects of derivatization chemistry and calibration strategy are presented. The method was validated by comparison with a standard method based on ion chromatography. The two independent methodologies produced results in agreement within 3%. Also a three level spike recovery test was carried out for validation purpose and quantitative recoveries were attained. The method is fast, simple, safe, and sensitive. Measurement of a 1 mL volume 50 ng/g of thiocyanate standard produced a signal-to-noise ratio of 250 for the analytical peak. This method is therefore suitable for ultra-trace determination of thiocyanate (low part-per-billion range). For the application described the full detection potential of the method was not required and the sample preparation presented has been designed for quantitation of saliva samples containing 1-400 μg/g of thiocyanate with a combined standard uncertainty of 2% relative for saliva samples containing 25 μg/g of thiocyanate. This method was applied for the determination of thiocyanate in human saliva samples.

Coordinate Swapping in Standard Addition Graphs for Analytical Chemistry: A Simplified Path for Uncertainty Calculation in Linear and Nonlinear Plots

Uncertainty of the result from the method of standard addition is often underestimated due to neglect of the covariance between the intercept and the slope. In order to simplify the data analysis from standard addition experiments, we propose x-y coordinate swapping in conventional linear regression. Unlike the ratio of the intercept and slope, which is the result of the traditional method of standard addition, the result of the inverse standard addition is obtained directly from the intercept of the swapped calibration line. Consequently, the uncertainty evaluation becomes markedly simpler. The method is also applicable to nonlinear curves, such as the quadratic model, without incurring any additional complexity.

Direct determination of dissolved phosphate and silicate in seawater by ion exclusion chromatography sector field inductively coupled plasma mass spectrometry.

A method is described for the direct determination of dissolved phosphate and silicate in seawater using ion exclusion chromatography (IEC) coupled with sector field inductively coupled plasma mass spectrometry (SF-ICPMS). Dissolved silicate was determined by double isotope dilution using a (29)Si spike, whereas one point gravimetric standard addition with internal standard of the same (29)Si spike was employed to quantitate dissolved phosphate. Medium resolution was used for all measurements in order to resolve polyatomic interferences on Si and P isotopes. Concentrations of 1.670 ± 0.008 and 30.20 ± 0.09 μM (SD, n = 6) with precisions of 0.47 and 0.31% for the dissolved phosphate and silicate, respectively, were obtained in National Research Council Canada certified reference material MOOS-3 seawater, in good agreement with certified values of 1.60 ± 0.15 and 30.5 ± 0.8 μM (U, k = 2), respectively. The reported method is a rapid (10 min per run), simple, and accurate online technique that requires no sample pretreatment. Moreover, this procedure achieves <0.5% precision (at above analyte concentrations) and method detection limits of 0.006 and 0.004 μM (0.18 as P and 0.11 ng g(-1) as Si), respectively, using a of 100 μL injection of seawater. The proposed technique is robust and well-suited for the determination of dissolved phosphate and silicate in seawater.

Condensation cascades and methylgroup transfer reactions during the formation of arsane, methyl- and dimethylarsane by aqueous borohydride and (methyl) arsenates

AbstractThe formation of volatile products during the reaction of As(iii), As(v), MeAsO(OH)2, and Me2AsO(OH) with aqueous NaBH4 has been investigated, and the formation of arsanes, diarsanes, and triarsanes has been detected. The presence of triarsanes is reported here for the first time. Diarsanes and triarsanes are likely formed in condensation cascade reactions, whereas trimethylarsane arises via the transfer of a methyl group. The formation of volatile by-products is considerably reduced by increasing the acidity of the medium and the concentration of NaBH4 or by the addition of thiols, such as cysteine. A reaction scheme is proposed which reconciles the evidence reported herein and elsewhere in the literature that is valid for both analytical (trace analysis) and non-analytical reaction conditions. FigureCondensation cascade and methyl transfer reactions taking place during the hydride generation of As-compounds under non-analytical conditions. They are originated by the interaction of As reaction intermdiates, among them, and with the other As species containing As-H and As-OH bonds.

Sub-ppt determination of butyltins, methylmercury and inorganic mercury in natural waters by dynamic headspace in-tube extraction and GC-ICPMS detection

A sensitive and accurate method is developed for the simultaneous ultra-trace determination of tributyltin (TBT), dibutyltin (DBT), monobutyltin (MBT), methylmercury (MeHg) and inorganic mercury (iHg) in waters including seawater by dynamic headspace in-tube extraction (dHS-ITEX) and GC-ICPMS detection. Quantitation of TBT, DBT, iHg and MeHg was achieved by isotope dilution mass spectrometry using 117Sn-enriched TBT and DBT, 201Hg-enriched iHg and enriched Me198Hg (NRC CRM EMMS-1), respectively, wherein analyte mass fractions in enriched spikes were determined by reverse isotope dilution at the same time using natural abundance TBT, DBT, iHg and MeHg primary standards by exactly matching the analyte and enriched spikes. Quantitation of MBT was realized by standard addition calibration with 117Sn-enriched DBT serving as the internal standard. The proposed method achieved detection limits of 0.06, 0.08 and 1.1 pg g−1 (as Sn) for TBT, DBT and MBT and 0.08 and 0.4 pg g−1 (as Hg) for MeHg and iHg, respectively. Validation of the proposed method was demonstrated by quantitative spike recoveries of 94–105% at 5 to 50 pg g−1 levels achieved in drinking water, river water and seawater, demonstrating the accuracy of the method. The obtained detection limits are sufficiently low to perform measurements in support of the Water Framework Directive. The developed method was applied for the determination of butyltins, iHg and MeHg in drinking water (AQUA-1), river water (SLRS-6) and seawater (CASS-6 and NASS-7) samples. Values of 0.124 ± 0.041 and 0.157 ± 0.060 pg g−1 (1SD, n = 4) for MeHg in CASS-6 and NASS-7 and values of 1.74 ± 0.61, 22.15 ± 0.13, and 31.42 ± 0.30 pg g−1 (1SD, n = 4) for iHg in SLRS-6, CASS-6 and NASS-7 were obtained, respectively. A value of 2.27 ± 0.45 pg g−1 as Sn for MBT was detected in NASS-7 seawater, whereas other butyltins were below the detection limits in the waters.